Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init()

Michal Hocko
2019-07-25T12:56:39Z

On Wed 24-07-19 16:30:17, David Hildenbrand wrote:
> We end up calling __add_memory() without the device hotplug lock held.
> (I used a local patch to assert in __add_memory() that the
> device_hotplug_lock is held - I might upstream that as well soon)
>
> [ 26.771684] create_memory_block_devices+0xa4/0x140
> [ 26.772952] add_memory_resource+0xde/0x200
> [ 26.773987] __add_memory+0x6e/0xa0
> [ 26.775161] acpi_memory_device_add+0x149/0x2b0
> [ 26.776263] acpi_bus_attach+0xf1/0x1f0
> [ 26.777247] acpi_bus_attach+0x66/0x1f0
> [ 26.778268] acpi_bus_attach+0x66/0x1f0
> [ 26.779073] acpi_bus_attach+0x66/0x1f0
> [ 26.780143] acpi_bus_scan+0x3e/0x90
> [ 26.780844] acpi_scan_init+0x109/0x257
> [ 26.781638] acpi_init+0x2ab/0x30d
> [ 26.782248] do_one_initcall+0x58/0x2cf
> [ 26.783181] kernel_init_freeable+0x1bd/0x247
> [ 26.784345] kernel_init+0x5/0xf1
> [ 26.785314] ret_from_fork+0x3a/0x50
>
> So perform the locking just like in acpi_device_hotplug().

While playing with the device_hotplug_lock, can we actually document
what it is protecting please? I have a bad feeling that we are adding
this lock just because some other code path does rather than with a good
idea why it is needed. This patch just confirms that. What exactly does
the lock protect from here in an early boot stage.

> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> Cc: Len Brown <lenb@kernel.org
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> drivers/acpi/scan.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> index 0e28270b0fd8..cbc9d64b48dd 100644
> --- a/drivers/acpi/scan.c
> +++ b/drivers/acpi/scan.c
> @@ -2204,7 +2204,9 @@ int __init acpi_scan_init(void)
> acpi_gpe_apply_masked_gpes();
> acpi_update_all_gpes();
>
> + lock_device_hotplug();
> mutex_lock(&acpi_scan_lock);
> +
> /*
> * Enumerate devices in the ACPI namespace.
> */
> @@ -2232,6 +2234,7 @@ int __init acpi_scan_init(void)
>
> out:
> mutex_unlock(&acpi_scan_lock);
> + unlock_device_hotplug();
> return result;
> }
>
> --
> 2.21.0

--
Michal Hocko
SUSE Labs